This application relates to and incorporates herein by reference Japanese Patent Applications No. 10-126292, 10-126293 and 10-126294, all being filed on May 8, 1998.
1. Field of the Invention
The present invention relates to a discharge lamp apparatus, which drives a high voltage discharge lamp and is preferably used as a vehicle front light.
2. Description of Related Art
Various discharge lamp apparatuses are proposed (e.g., JP-A-9-180888 (U.S. Pat. No. 5,751,121) and JP-A-8-321389), which use a high voltage discharge lamp (lamp) as a vehicle front light, drives the lamp by alternating current (a.c.) voltage after boosting a voltage of a vehicle-mounted battery by a transformer and switching the polarity of the high voltage by an inverter circuit.
This lamp is mounted inside of a reflector provided at a vehicle front part. When an electric wiring part of the lamp is grounded accidentally, an excessive current flows and melts a fusible link or damage circuit devices in the discharge lamp apparatus.
Further, a switching device is provided at a primary side of a voltage boosting transformer to control a primary current, and controls electric power supplied to the lamp by pulse width modulation (PWM) control based on a lamp voltage and a lamp current. In this PWM control, when the duty ratio is increased to increase the electric power of the lamp, the secondary side output of the transformer decreases oppositely. Therefore, a maximum duty ratio is set to limit the duty ratio to be less than a maximum.
However, if the maximum duty ratio is set as above, the lamp can not be supplied with sufficient electric power when the lamp does not continue to light because of decrease in the lamp current at the time of starting lighting the lamp.
Still further, in the above discharge lamp apparatus, an electronic unit for the lamp is encased within a ballast housing, and the ballast housing is mounted outside of the lamp. Thus, extra space is required at the outside of the lamp for installing the electronic unit.
It is a primary object of the present invention to improve operation characteristics of a discharge lamp apparatus.
More specifically, the present invention aims to improve fail-safe operation when an electric wiring part of a lamp is grounded, to improve lighting characteristics of a lamp, or to improve mountability of a starter transformer in a lamp.
According to one aspect of the present invention, it is determined to be a grounded condition when a voltage between a transformer and an inverter circuit is less than a predetermined voltage and a current flowing to a negative side of a d.c. voltage source is less than a predetermined current. At this occasion, electric power supply to a discharge lamp is stopped temporarily by turning off a plurality of switching devices in an inverter circuit. Thereafter, the electric power supply is restarted by the plurality of the switching devices.
When the grounded condition is determined again after starting the electric power supply, the electric power supply is repeatedly stopped and started. All the plurality of the switching devices are held turned off, when the repetition of stopping and starting of the electric power supply continues for a predetermined period of time.
According to a second aspect of the present invention, an upper limit value is set for a duty ratio of a switching device connected to a primary side of a transformer. This upper limit is varied by a battery voltage, lamp voltage, and a lamp current flowing in a lamp. The upper limit increases as the current decreases. Thus, the secondary side output of the transformer can be increased sufficiently to improve the lighting characteristics of the lamp.
According to a third aspect of the present invention, a ballast casing encasing a starter transformer is mounted in a lamp. A cross sectional area S of a closed magnetic circuit core of the starter transformer and an inside height H of the ballast casing are determined to satisfy a relation of Hxe2x89xa6xe2x88x920.0015 S2+0.54xc2x7Sxe2x88x9211.49. A gap of the core is located at the central part side in the ballast casing.